Encased electrical apparatus having pressure relief means



P 15, 1959 J. F. KOEPKE ET AL 2,904,616

ENCASED ELECTRICAL APPARATUS HAVING PRESSURE RELIEF MEANS Filed March 18. 1958 3 Sheets-Sheet l I NVE NT ORS. Ja/wv A KOEPKE JOHN J." z/Ms/o way/2M A. Mar/(5Q Sept. 15, 1959 KOEPKE ETAL' 2,904,616

ENCASED ELECTRICAL APPARATUS HAVING PRESSURE RELIEF MEANS 5 Sheets-Sheet 3 Filed March 18, 1958 atent fihce 2,904,616 iatented Sept. 15, 1959 ENCASED ELECTRIEAL APPARATUS HAVING PRESSURE RELIEF MEANS John F. Koepke, Bethel Park, and John J. Zimsky and William A. M'oclker, Bridgeville, Pa., assignors to McGraw-Edison Company, Milwaukee, Wis., a corporation of Delaware Application March 18, 1953, Serial No. 722,324-

15 Claims. (Cl'. 1741-1 This invention relates to electrical apparatus. and particularly to encased electrical apparatus having overpressure protection.

High voltage electrical apparatus is often immersed in an insulating and cooling fluid within a sealed casing to increase the voltage breakdown strength and to improve cooling, and means are provided to relieve pressure which may be built up within the casing due to arcing or overload. While the invention embraces any such encased electrical apparatus, it will be described by way of example with respect to oil-filled electrical power transformers which often have their core and coils immersed in oil within a sealed casing. Because the temperature expansion coefficient for transformer oil is appreciable, the oil should be permitted to expand and contract with variations in transformer temperature. This is often accomplished by filling the space above the oil with a cushion of inert gas such as nitrogen which is compressible and allows the oil to expand. In the event of an are under the oil due to breakdown of the oil, a gas is evolved which is. approximately 70 percent, hydrogen and which when mixed with oxygen may give a violent explosion, called the secondary explosion. Further, the gas is evolved at a very rapid rate-approximately 100 cubic centimeters per kilowatt-second of are energyand the effect of the gas evolution, called the primary explosion, may be so rapid as to break open gasketed devices such as bushings, handhole covers, and switch or terminal compartments and even rupture the casing before prior art pressure relief devices could operate to exhaust the gases. Conventional transformer casings, which are generally designed to withstand an internal pressure in the order of lbs. per sq. in., may be ruptured or permanently deformed by the. rapid gas evolution. Further, such explosions introduce the hazard of fire. Consequently, it is desirable to provide oil-filled transformers with over-pressure protection devices whose rate of response is sufficiently rapid to exhaust the casing before the rapidly increasing pressure of the evolved gas exceeds the value which the casing can safely withstand, and which also respond to excessive pressure resulting from continuous overload.

Prior art pressure relief devices generally include pressure responsive means which must be reset or replaced after operation and may take the form of a frangible member such as a diaphragm sealing an aperture in the transformer casing and adapted to be ruptured at a predetermined pressure within the casing, thereby allowing the evolved gases to be exhausted. The internal pressure within the casing at which the frangible diaphragm fractures can seldom be accurately predetermined because of such variables as non-uniform clamping pressure on the sealing gaskets, expansion and contraction of the frangible material, e.g., glass, with variations in temperature, and failure of the operator to correctly center the frangible glass diaphragm disk before assembly. After these prior art pressure relief devices operate, it is necessary to deenergize the transformer with consequent interruption of customer service. Further, it is necessary to replace the frangible sealing member or reset the pressure responsive means, thereby increasing maintenance costs and the expenses incident to transformer outages. Nitrogen above the oil escapes upon operation of such prior art pressure relief devices, thereby requiring re-charging of the casing with nitrogen after resetting. The aperture in the transformer casing opened by operation of a prior art pressure. relief device allows entry into. the casing of moisture and other foreign matter which may contaminate and lower the dielectric strength of the oil and also cause rusting within the tank. This. condition. is particularly aggravated if the transformer is within an underground vault which may be flooded at the time the prior art pressure relief device operates, thereby permitting water to pour into the transformer casing. After prior art pressure relief devices have operated, the pressure within the casing decays to. atmospheric pressure, thereby allowing gases dissolved in the. transformer oil to form bubbles and thus ad'- versely affect the dielectric properties of the oil. Further, oxygen can enter through the opening in the casing after internal pressure has. been relieved and form explosive mixtures withgases being evolved, thereby creating a serious fire hazard.

It is an object of the invention to provide electrical apparatus; having a new and improved pressure relief means.

7 It, is. a further object of the invention to provide encased. electrical apparatus having pressure relief means which automatically reseal the casing when the pressure within the. casing returns to a safe value.

Another object:- of the, invention is to provide encased fluid cooled electrical apparatus having pressure relief means provided with a suflicientlylarge opening to permit substantially instantaneous venting of gases and other fluids. under pressure within the casing and which responds with sufficient rapidity to exhaust the casing before the rapidly evolved gases can damage the casing.

It is a further object of the invention to provide encased electrical apparatus having pressure responsive means wherein increase of pressure within the casing to a predetermined value triggers a greater force actuating, the pressure responsive means to vent the casing to the atmosphere.

Still another object of the invention is to provide encased electrical apparatus having pressure relief means which does not require maintenance, resetting or replacement after operation.

A. still further object of the invention is to provide encased electrical apparatus having pressure relief means which allows the apparatus to remain in service after the relief means has vented the casing.

It is a further object of the invention to provide encased, hermetically sealed, oil-filled electrical apparatus having pressure relief means which immediately reseals after venting the casing, thereby preventing entrance of dust and moisture into the casing which might contaminate the oil.

A further object of the invention is to provide an encased, hermetically sealed, oil-filled electrical apparatus having pressure relief means which reseal under a positive internal pressure, thereby preventing gases dissolved in the oil from coming out of solution because of drop in pressure.

t is another object of the invention to provide an encased, hermetically sealed electrical apparatus having pressure relief means sensitive to small variationsv of internal casing pressure and which can be accurately set so that operation always occurs within limits which are very close to the desired predetermined operating pressure.

It is a still further object of the invention to provide encased, fluid-cooled electrical apparatus having pressure relief means which prevent entry of water or moisture into the casing upon operation of the pressure relief means even if the entire casing is submerged beneath water in an underground vault.

Still another object of the invention is to provide encased, fluid-cooled electrical apparatus having pressure relief means which reseals against a positive internal pressure so that no oxygen from the atmosphere can enter the casing, whereby the danger of fire caused by hydrogen-oxygen secondary explosions is eliminated.

It is an object of one embodiment of the invention to provide encased electrical apparatus having pressure relief means which can be calibrated and tested as a separate unit independent of the casing.

These and other objects and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein:

Fig. 1 is an elevation view, partly in section, of a preferred embodiment of stationary induction apparatus embodying the invention;

Fig. 2 is a sectional view of one embodiment of pressure responsive means included in the apparatus of Fig. l, the pressure responsive means being in closed position;

Fig. 3 is a view similar to Fig. 2 showing the pressure responsive means in open position;

Fig. 4 is an enlarged sectional view of a portion of the pressure responsive means of Fig. 2;

Fig. 5 is a sectional view of an alternative embodiment of pressure responsive means wherein no cover movement is required to trigger the device; and

Fig. 6 is an enlarged sectional view through the pressure bleeder of the embodiment of Fig. 5.

Although the invention comprehends the combination of any encased electrical apparatus immersed in an insulating and cooling fluid with means for relieving pressure within the casing due to overload or arcing beneath the fluid, the invention will be described as preferably embodied in oil-filled stationary induction apparatus having a gas cushion above the oil. Referring to the drawing, Fig. 1 illustrates an electrical transformer having a casing 10 hermetically sealed by a cover 11 secured thereto by suitable means (not shown). A transformer core and coil assembly including a magnetic core 12 linked by a coil 13 and positioned within the casing 10 is immersed in an insulating and cooling fluid 14 such as transformer oil or a halogenated, oxidation-resistant askarel liquid which fills a portion of the casing 10. Au inert gas 15 such as nitrogen fills the remainder of the casing 10 above the cooling fluid 14 and allows the cooling fluid 14 to expand. Electrical connections are made to the electrical coil 13 through insulating bushings 16 mounted on and extending through casing cover 11. Pressure relief means 18 mounted on casing cover 11 normally closes an aperture 19 in the casing cover 11. However, it will be appreciated that the aperture 19 may be in any portion of the casing 10 which will place the pressure responsive means 18 in communication with the gas 15. Although the invention is illustrated as embodied in a transformer having a gas cushion and pressure responsive means in communication with the gas, the invention is also suitable for exhausting gases or liquids under pressure in encased transformers provided with conservators.

One embodiment of pressure relief means 18 shown in closed position in Figs. 2-4 includes a cylindrical body portion 20 having an axial opening 21 in register with aperture 19 in casing cover 11 and a pressure responsive member, or cover, 22. The body portion 20 has an outwardly extending annular flange 23 at its lower end provided with a plurality of circumferentially spaced apertures 24 for receiving bolts 25 which are threaded within suitable tapped holes in the casing cover 11 to compress a resilient annular gasket 26 between the body portion 20 and casing cover 11. The pressure responsive member, or cover, 22 is in the shape of an inverted saucer and has an enlarged central portion 28 and a pair of concentric, downwardly extending, annular flanges 29 and 30 which divide the lower surface of pressure responsive member 22 into an annular face 32 radially inward from flange 29 and partially defining an inner area on cover 22 and an annular face 33 radially outward from flange 29 and forming an outer area on cover 22. Both faces 32 and 33 are disposed toward the interior of casing 11 and generally transverse of the direction of reciprocation of cover 22.

When cover 22 is in closed position first sealing means surrounding the inner area, including a pair of opposed continuous surfaces, one of which is on cover 22 and the other is on body portion 20, seal the opening 21 and isolate the outer area from the interior of the casing 10 and second sealing means surround both the inner and outer cover areas and include a pair of opposed continuous surfaces, one of which is on cover 22 and the other of which is on body portion 20. In the embodiments shown in the drawing, such pairs of opposed continuous surfaces are formed by pairs of telescopingly overlapped sealing surfaces on body portion 20 and pressure responsive member 22 in piston-and-cylinder relation, but it will be appreciated that this invention also comprehends embodiments wherein the opposed surfaces are not telescopingly overlapped.

In order to provide rapid opening of cover 22, it is desirable that an effective seal be retained at the outer sealing means surrounding both areas on cover 22 after both areas are exposed to internal pressure within casing 10. However, it is also desirable to provide restricted communication between the outer area and the atmosphere when the cover 22 is in closed position to assure that no initial force will be exerted on the outer area due to gas trapped under pressure between the inner and outer sealing means. This means for providing restricted communication between the outer area on cover 22 and the ambient can be accomplished by spacing apart the opposed continuous surfaces of the second sealing means, but in the preferred embodiments these opposed surfaces are in sealing engagement and bleeder holes 40 provide the restricted passageway as explained in detail hereinafter.

The pairs of telescopingly overlapped sealing surfaces are provided in the embodiments shown in the drawing by the concentric depending flanges 29 and 30 on the pressure responsive member 22 and the vertical surfaces at the inner and outer peripheries of body portion 20 opposed to said flanges. Specifically, the outer periphery at the upper end of the body portion 20 forms one sealing surface 34 and is provided with a trapezoidal-in-cross section annular groove 35 in which an annular, O-ring, resilient gasket 36 is disposed. The use of gasket 36 within groove 35 assures that the gasket material cannot be overcompressed and thus cause the material to assume a permanent set as frequently occurred in prior art devices wherein the entire cross section of the gasket was compressed between opposing surfaces. The gasket 36 is preferably of a suitable material such as silicone rubber which is inert to transformer oil and will not lose its resiliency even when exposed to the atmosphere for many years.

The inner periphery of outer depending flange 30 on pressure responsive member 22 has an annular sealing surface 38 parallel to and opposed to sealing surface 34 on body portion 20, the flange 30 being so proportioned with respect to the exterior of the body portion 20 that surface 38 on pressure responsive member 22 is slidable past surface 34 on body portion 20, thereby compressing gasket 36 within annular groove 35 to nor- .from the atmosphere.

family seal face 33 defining the outer area on cover 22 In the embodiment illustrated in Figs. 2-4, a plurality of bleeder holes 40 are provided through the outer flange 30 in circumferentially spaced apart relation around the periphery of the cover 22 to vent the outer area on cover 22 to the atmosphere. The lower edge of surface 38 is chamfered at 39 to permit the surface 38 to slide past gasket 36 with little or no tendency to urge it out of groove 35.

The upper end of the body portion 20 has a continuous, radially inward extending flange 41 which terminates at its inner end in an enlarged annular rim 42 having an annular sealing surface 44 parallel to the direction of reciprocation of cover 22 and which defines a gas exhaust aperture 27 through which the casing is vented. The exterior periphery of the inner depending flange 29 on pressure responsive member 22 forms an annular sealing surface 45 parallel and normally opposed to sealing surface 44 on rim 42. It will be understood that if the inner flange 29 is of appreciable width as shown in Fig. 4, the lower face thereof constitutes a part of the inner area on cover 22 which is normally exposed to internal pressure within casing 10. A trapezoidal-in-cross section annular groove 46 formed in sealing surface 45 receives an annular O-ring gasket 48 of suitable resilient material such as silicone rubber which is inert to transformer oil and does not lose its resiliency when exposed to the atmosphere. The O-ring gasket 43 fitting with groove 46 assures that the gasket cannot be overcompressed and thus be permanently deformed as frequently occurred with prior art sealing means wherein the entire cross section of the gasket was compressed between opposed surfaces. The trapezoidal cross section of grooves 35 and 46 is particularly advantageous in retaining gaskets 36 and 48 within the grooves when the opposed sealing surfaces are moved past each other during the opening and closing of cover 22. Inner flange 29 on pressure responsive member 22 and the inwardly extending flange 41 of body portion are so proportioned that opposed sealing surfaces 44 and 45 are slidable past each other to compress gasket 48 within groove 46 and thus seal the interior of casing 10 from the atmosphere and also to isolate the outer area on pressure responsive member 22 from the interior of casing 10 when the pressure responsive member 22 is in the closed position. The gas exhaust aperture 27 in body portion 20 provides a seat for the inner area on pressure responsive member 22. The upper edge of rim 42 is chamfered above surface 44 to permit surface 44 to slide past gasket 48 with little or no tendency to urge it out of groove 46. While the invention has been illustrated and'described with the body portion 20 as a member separate from the casing 14), it will be appreciated that it may be formed as an integral part of the cover 11 or casing 10.

The pressure responsive member 22 is biased toward closed position by a compression spring 60 circumjacent an elongated bolt 61. The upper end of bolt 61 is threadably engaged within an axial opening 63 in the enlarged central portion 28 of the cover 22. The lower end of spring 60 bears against a washer 67 disposed above a nut 68 threaded on the lower end of the bolt 61. A plurality of circumferentially spaced apart, radially inward extending, supporting ribs 69 integrally attached at their outer end to the interior periphery of body portion 20 are integrally affixed at their inner ends to an axial hub 70. The hub 70 has an axial bore 72 slidably receiving bolt 61, and a sleeve bearing 73 of suitable material such as oil-impregnated porous bronze commercially available under trademark Oilite, may be positioned within axial bore 72 wherein it is supported by an inwardly extending flange 74 at the lower end of the hub '70. The upper end of spring 60 bears against flange '74 on hub 70. It can thus be seen that the spring 60 will urge the pressure responsive member 22 toward closed position wherein the lower end of the enlarged cen- 6 tral portion 28 is in direct metal-to-metal engagement with the upper end of the hub 70, thereby providing a positive mechanical stop for member 22 and bringing the pair of opposed sealing surfaces 34 and 38 and the pair of opposed sealing surfaces 44 and 45 into telescoped overlapping relation with gaskets 36 and 48, respectively, compressed therebetween. The biasing force of the spring can be varied as desired by turning nut 68 to change the length of the compressed spring 60 but in the preferred embodiment the force exerted by spring 66 is suflicient to retain portion 28 in metal-to-metal contact with hub over the normal range of operating pressures within casing 10, thereby preventing continuous fluctuation in the position of cover 22 and mechanical working of gaskets 36 and 48.

Triggering of the pressure responsive means to expose both areas on cover 22 to the pressure within casing 10 is accomplished in the embodiment of Figs. 2-4 by the force resulting from internal casing pressure displacing the cover slightly in an outward direction to open the inner sealing means, Whereas in the embodiment of Figs. 5 and 6 no movement of cover 22 is required to trigger the pressure relief means and expose both areas to pressure within casing 10. In the embodiment of Figs. 2-4 the overlap between the opposed surfaces 44 and 45 sealing the outer area on pressure responsive member 22 from the interior of casing 13 is preferably less than the overlap between the opposed sealing surfaces 34 and 38. Consequently, the displacement of the pressure responsive member 22 necessary to expose the outer face 33 to the internal pressure within casing 10 is less than that required to vent the interior of casing 10 to the atmosphere. Stated in another manner, when the cover 22 is in closed position, the gasket 36 is a greater distance from the lower edge of sealing surface 38 than the gasket 48 is from the upper edge of sealing surface 44. Consequently, upon displacement of the cover 22 away from closed position, the extent of travel of cover 22 necessary to disengage gasket 48 from surface 44 is less than that required to disengage surface 38 from gasket 36, and both the inner and the outer areas of the cover 22 are exposed to internal pressure within casing 10 before the casing 10 is vented to the atmosphere. it is to be understood that, if desired, the pairs of opposed, telescopingly overlapped sealing surfaces 3438 and 44-45 may be machined to sufiiciently close tolerances so hat a seal is established directly therebetween without the use of gaskets 36 and 48 or so that the separation between surfaces 34 and 38 could form the means to provide restricted communication between the outer area on cover 22 and the ambient.

When cover 22 is in closed position as shown in Fig. 2, it is subject to an outward force equal to the internal pressure within the casing multiplied by the inner area of cover 22, which inner area is defined by face 32 and the lower surface of inner flange 29. This outward force is opposed by the downward force exerted by spring 60 against the lower end of bolt 61. The spring 60 is calibrated so that it will be compressed sufficiently to open the inner seal at gasket 43, but not the outer seal at gasket 36, when spring 60 is subjected to a force equal to that resulting from the inner area of cover 22 being exposed to a predetermined desired pressure which is less than that which will rupture the casing 10. This outwardly directed force causes cover 22 to be displaced a short distance relative to the body member 20 to the position shown in Fig. 4 wherein gasket 43 is above surface 44 on rim 42, thereby lifting the inner area portion of the cover 22 ofl the seat defined by the gas exhaust aperture 27 and exposing the outer area (defined by surface 33) on cover 22 to the pressure within the casing 10. However, because the force required to compress a spring is proportional to deflection, the initial displacement of pressure responsive member 22 may not be sufficient to move surface 38 out of overlapping engagement with gasket 36 and surface 34. Consequently, the

casing 10 is not exhausted to the atmosphere by the initial displacement of cover 22. During the initial upward movement of cover 22, bleeder holes 40 allow air to be drawn into the space below surface 33 so that a partial vacuum will not be created therein by movement of cover 22 which would tend to close cover 22. In the instant after the inner seal is broken at gasket 48, the gas within the casing escapes between the inner flange 29 and the rim 4-2, as shown by the upwardly directed arrows in Fig. 4, to expose face 33 to the internal pressure within the casing 10. Consequently, the upward force acting on cover 22 will be instantaneously increased by an amount equal to the outer area on pressure responsive member 22 multiplied by the internal pressure within the casing Ill. As soon as the pressure responsive means is triggered by breaking of the seal at gasket 48, the upward force acting on cover 22 becomes larger than that necessary to compress spring 60, and the pressure responsive member 22 pops up toward an open position shown in Fig. 3 wherein casing 10 is vented to the atmosphere, thereby rapidly exhausting casing 11. The gas exhaust aperture 27 is sufficiently large to provide substantially instantaneous venting to the atmosphere and prevents build-up of gases evolved from arcing beneath the oil which in prior art devices often ruptured the transformer tank and started fires. The depending portion 76 at the lower edge of outer flange 30 deflects the escaping gases downward to assure that the escaping gases react with maximum efiiciency against cover 22 so that it will remain open until desired relief of the evolved gases or liquid under pressure is accomplished. After the pressure within casing 10 has returned to a second predetermined pressure considerably below the predetermined triggering pressure required to trigger the pressure responsive means 18, spring 60 will force cover 22 back to the closed position, thereby reestablishing the seals around the inner and outer areas of the cover 22.

Inasmuch as the pressure within casing 10 reacts against both areas on the cover 22 when the cover is away from closed position, the second predetermined pressure at which cover 22 recloses will bear the same relation to the predetermined triggering pressure as the inner area bears to the entire cover area. For example, if the inner area is thirty square inches and the total area is sixty square inches and the pressure responsive means is set to operate at ten pounds per square inch, the spring 60 Will be calibrated to exert an initial force of (30 sq. in. times 10 lbs. per sq. in. equals) 300 pounds, and the cover 22 will return to closed position when the pressure within casing 10 returns to (300 lbs. divided by 60 sq. in. equals) lbs. per sq. in. It will be appreciated that the cover 22 reseals against a positive internal pressure within casing 22, and that the outward flow of gases under pressure positively prevents entrance into the casing 1 of water or moisture which might cause rusting within the casing and deleteriously affect the dielectric strength of the oil. Tests prove that the pressure responsive means of the invention prevents entry of water into the casing It) even when the entire casing is submerged beneath a pressure head of Water as may occur when the casing is installed in a flooded underground vault, whereas prior art devices having pressure relief means of the frangible disc or the mechanical latch type were unsuitable for use in underground faults.

The internal pressure within electrical apparatus having prior art pressure relief means of the frangible disc 'or mechanical latch type dropped from a relatively high triggering pressure, e.g., 10 lbs. per sq. in., to atmospheric pressure almost instantaneously when the pressure relief device operated. It is well known that a rapid :decrease of pressure causes formation of free gas bubbles within insulating oil which materially decreases the dielectric strength of the oil. It will be appreciated that a relatively high positive predetermined pressure is maintained within casing 10 when cover 22 recloses, and that consequently the danger of lowering the dielectric strength of the oil is obviated. Further, the entrance of oxygen into the casing through the opening provided by rupture of the frangible disc of prior art pressure relief means created the danger of fire caused by secondary explosions from hydrogen-oxygen mixtures, whereas the cover 22 resealing the casing 10 against a positive internal pressure within casing 10 assures that no oxygen can enter the casing and thus eliminates the hazard of fire caused by secondary explosions.

The bleeder holes 40 provide a restricted passageway between the outer area on cover 22 and the ambient and allow any gases trapped under face 33 to vent to atmosphere so that these gases cannot be compressed and thereby exert an initial upward force on cover 22 which would reduce the internal pressure required to actuate pressure responsive member 22 in subsequent operations thereof. These bleeder holes 40, however, are sufficiently small so that they do not have any substantially effect upon the instantaneous actuation of pressure responsive member 22 toward open position.

In order to give an indication that the pressure responsive means has operated, an alarm actuating means 84 is mounted on cover 11 adjacent the body portion 20. The alarm actuating means 84 includes an enclosed electrical switch 86 mounted by suitable means such as screws 87 upon an upstanding bracket 88 which is afiixed to cover 11 by a bolt 89 and a lever 90 pivotally mounted on bracket 88 by means of pin 91. The switch 86 encloses a pair of contacts (not shown) normally actuated to open position by resilient means (not shown) which also urge a contact actuating member 94 outward through an opening in the casing of switch 86. When the alarm actuating means is in the normal, non-indicating position, one end 92 of the lever 90 rests on the upper side of the pressure responsive member 22 while the other end 93 is in the path of and holds contact actuating member 94 in its inward position wherein the contacts of electrical switch 86 are open. When the pressure responsive member 22 pops up toward an open position, lever 90 is pivoted about pin 91 to the position shown in Fig. 3 wherein end 93 strikes stop member 95 integral with bracket 88. This movement is sutficient to release contact actuating member 94 for outward movement under spring force and thus permit closure of the contacts of electrical switch 86. Operation of switch 36 in turn causes a remote signal, shown schematically in Fig. 1 as a lamp 96, to be energized in a manner well known in the art, and the lamp will continue to indicate that the pressure responsive means has operated after cover 22 has returned to closed position. The alarm actuating means 84- can be manually reset by engaging lever 9t? with a hookstick from the exterior of the casing without de-energizing the electrical transformer.

As explained hereinbefore, when the internal pressure Within casing 10 falls below the value which, when exerted against the total cover area, results in a force less than the spring force, spring 60 will return cover 22 to closed position wherein seals are again effected at gaskets 36 and 48, thus obviating the necessity of resetting the mechanism and replacing frangible diaphragrns after operation as were required with prior art pressure relief devices. It is unnecessary to de-energize the transformer to reset the disclosed pressure responsive means after operation, and consequently there is no interruption of customer service as frequently occurred with prior art devices which required resetting and replacing of fragible members. Inasmuch as cover 22 assumes a plurality of open positions depending upon pressure Within casing 10, the size of the effective opening for exhausting the evolved gases decreases with pressure but the velocity of the escaping gases remains approximately constant regardless of the displacement of the cover. Consequent- 9 1y no moisture and foreign matter can eriter the casing against the force of the escaping gas where it might contaminate and lower the dielectric strength of the transformer oil. The rescaling of the pressure responsive member 22 under a positive internal pressure not only prevents entrance of foreign matter into the casing but also prevents formation of free gas bubbles in the transformer oil, due to a relatively large pressure drop, which may adversely affect the dielectric properties of the oil. It will also be noted that the entire pressure responsive means is supported on body portion 20 which can be removed from the casing to permit testing and calibration of the pressure responsive means independent of the transformer casing 10.

The double area cover 22 has the additional advantage over a single area pressure relief device that the cover 22 recloses at a second predetermined pressure sufiiciently below the triggering pressure to permit exhausting of gases and reduction of pressure within the casing below a dangerous value but still permits reclosing against a positive pressure within the casing so that the velocity of the gases escaping under pressure prevents entry into the casing of water, moisture, oxygen and other foreign matter.

The pressure responsive means of an alternative embodiment of the invention shown in Figs. 5 and 6 may be triggered without movement of the cover. Many of the components of this embodiment are identical with elements of the embodiment of Figs. 2 and 4 and are designated by the same reference numbers to which a has been added, and the description of these parts will not be repeated. The body portion 20' has a plurality of circumferentially spaced apertures for receiving bolts 110 which pass through suitable holes in an outwardly extending flange 111 of a tubular member 112 extending through and rigidly affixed by suitable means to the cover of the transformer. Casing bolts 110 are secured by nuts 113 to hold flanges 23 and 111 in metal-to-metal contact with gasket 114 compressed within annular recess 115 in the top surface of flange 111. In this embodiment surface 34' perpendicularly intersects the upper surface on body portion 20 to provide a mechanical stop for a land 116 in the surface 33' of cover 22'. The means for providing restricted communication between the outer area on the cover 22 and the ambient comprises a plurality of circumferentially spaced bleeder holes 117, similar to bleeder holes 40 of the embodiment of Figs. 2 and 4, extending through body portion 20 between surface 34' and the upper surface of inwardly extending flange 41'. The axial bore in tube 70 slidably receives bolt 61 without a sleeve bearing therebetween as provided in the embodiment of Figs. 2 and 4. Although the bolt 61 in the embodiment of Figs. 24 has a shoulder which abuts against the enlarged central portion 28, no such shoulder is provided on the bolt 61' and preferably the bolt 61' is not bottomed within the female thread in central portion 28 in order to permit some freedom of movement of bolt 61 relative to cover 22' due to clearance in the threads. Further, as can be seen in Fig. 5, slight clearance is provided between bolt 61 and the axial bore in hub 7 0 in which the bolt is received. This construction provides relatively loose coupling between cover and casing and permits the cover 22' to be at least partially self-aligning, thus assuring that both gaskets 36 and 48 are compressed to the same degree and also vassuring that bolt 61' does not bind in the bore in hub 70'. A pressure responsive bleeder member 120 shown in greater detail in Fig. 6 is mounted so as to extend through flange 41 and includes a body portion 121 having an axial bore 122 therethrough and a pressure responsive member 123 disposed in said bore. The body portion 121 {has an enlarged head portion 124 at its upper end and a threaded stem 125 engaged within a suitably tapped hole 126 in the inwardly extending flange 41'. The up per end of bore 122 is formed with a frusto-conical surface 128 whose minordiameter is smaller than that of the bore to provide a lip 129 extending radially inward from the inner surface of the bore 122. Pressure responsive member 123 has ahead portion 130 and a threaded stem portion 132 and the head portion 130 is provided with a conical surface 131 complementary to the surface 128 on the body portion 121 and which .cooperates therewith when the pressure responsive member 123 is in a closed position to seal the bore 122. Pressure responsive member 123 is biased toward closed position by a helical compression spring 134 surrounding stem portion 132. The ends of spring 134 bear against a nut threaded onto the lower end of stem portion 132 and lip 129 on the body portion 121. The biasing force of spring 134 can be varied as desired by turning nut 135 to change the initial compression of spring 134. An O-ring gasket 138 is disposed in an annular groove 139 in the conical surface 131 to normally seal between body portion 121 and pressure responsive member 123.

When cover 22 is in closed position, it is subject to an upward force equal to the pressure in the transformer casing multiplied by the inner area on cover 22, and this force is opposed by spring 60'. The spring 60 is so calibrated that when the pressure within the casing reaches a predetermined value, it balances the upward force exerted on the cover 22'. Spring 134 of the pressure bleeder 120, however, is so calibrated that when this predetermined pressure is reached within the casing, the

upward force exerted on pressure responsive member 123 of bleeder 120 overcomes the biasing force exerted by spring 134, thereby forcing pressure responsive member 123 away from the closed position and exposing the outer area on cover 22 including surface 33 to the pressure within the casing. The upward force on cover 22' then becomes equal to the unit pressure within the casing times the combined inner and outer areas. This force is greatly in excess of the downward force exerted by spring 60, thereby causing pressure responsive member 22' to pop up. When the unit pressure within the casing exerted against both areas on cover 22' drops to a value wherein the force directed outwardly against cover 22' is less than the biasing force of spring 69, the cover 22 will return to the closed position. The total area of bleeder holes 117 is sufliciently small relative to that of the bore 122 through the bleeder 120 that the amount of leakage through this restricted passageway will not materially reduce the pressure acting on surface 33 when the bleeder member 123 is in open position.

While the invention has been described as embodied in an oil filled electrical transformer, it is to be understood that the invention also embraces both other electrical apparatus and such apparatus having other types of insulating and cooling fluids such as high dielectric strength gases and halogenated, oxidation-resistant insulating liquids.

While only a few embodiments of the invention have been illustrated and described, many modifications and variations thereof will be obvious to those skilled in the art, and consequently it is intended in the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the invention.

We claim:

1. In electrical apparatus, in combination, a casing having an aperture therein, a cooling and insulating fluid within said casing, spaced apart portions of said apparatus being at different electrical potentials and being subject to arcing therebetween under fault conditions which may rapidly develop suflicient pressure within said casing to damage said casing, said aperture being of sufficient size to vent said casing before it is damaged from pressure incident to a large kva. arc, pressure responsive means including a double area cover for said aperture, said aperture providing a seat for one of the areas of said cover, said one area being in communication with the interior of said casing and the other area being isolated therefrom when said cover is closed, said means being responsive to a predetermined internal pressure within said casing to expose both areas of said cover to the internal pressure within said casing, resilient means for holding said cover in closed position, movement of said cover away from said closed position against the force of said resilient means permitting escape of gas from within said casing, said cover being returned to its closed position by said holding means after the pressure within said casing drops to a second predetermined value.

2. In electrical apparatus, the combination of a casing having an opening therein, a cooling fluid in said casing,

spaced apart portions of said apparatus being at different electrical potentials and being subject to arcing therebetween under fault conditions which may rapidly develop suflicient pressure within said casing to damage said casing, said opening being of suflicient size to vent said casing before it is damaged from pressure incident to a large kva. arc, a pressure responsive member mounted in said opening for displacement in response to the pressure in said casing and sealing said opening when in closed position, and resilient means biasing said pressure responsive member into closed position, said pressure responsive member being actuated away from said closed position against the force of said biasing means in response to a predetermined pressure within said casing, said biasing means returning said pressure responsive member to said closed position when the pressure in said casing is relieved.

3. In electrical apparatus, in combination, a casing having an aperture therein, a cooling and insulating fluid within said casing, spaced apart portions of said apparatus being at different electrical potentials and being subject to arcing therebetween under fault conditions which may rapidly develop suflicient pressure within said casing to damage said casing, said aperture being of sufficient size to vent said casing before it is damaged from pressure incident to a large kva. are, a cover for said aperture having first and second areas, sealing means surrounding said first area of said cover for sealing between said cover and said casing when said cover is in closed position, said first area being exposed to the interior of said casing through said aperture and said second area of said cover being isolated from the interior of said casing by said sealing means when said cover is in closed position, said cover cooperating with said casing to provide a restricted passage between said second area and the ambient when said cover is in closed position, resilient means for biasing said cover into closed position, said cover being displaceable away from said closed position against the force of said resilient biasing means in response to a predetermined pressure within said casing and the displacement of said cover necessary to unrestrict said passage and fully vent said second area to the ambient being greater than that required to open said sealing means, whereby both areas of said cover are exposed to the internal pressure with the said casing and said cover is rapidly actuated toward the open position wherein said casing is vented through said aperture.

-4. In electrical apparatus, in combination, a. casing having an aperture therein, a cooling and insulating fluid within said casing, spaced apart portions of said apparatus being at different electrical potentials and being subject to arcing therebetween incident to a failure which may rapidly develop sufiicient pressure within said casing to damage said casing, said aperture being of suflicient size to vent said casing before it is damaged from pressure incident to a large kva. arc, pressure responsive means including a cover for said aperture having first and second areas, sealing means surrounding said first area of said cover for sealing between said cover and said casing when said cover is in closed position, said first 12 4 area being exposed to the interior of said casing through said aperture and said second area of said cover being isolated from the interior of said casing by said sealing means when said cover is in closed position, means surrounding both areas on said cover for providing a restricted passageway between said cover and said casing when said cover is in closed position, resilient means for biasing said cover into closed position, said pressure responsive means being responsive to a predetermined pressure within said casing to expose the second area on said cover to the internal pressure within said casing, whereby said cover is rapidly actuated away from closed position and said casing is vented through said aperture.

5. In electrical apparatus, in combination, a casing having an aperture therein, an insulating and cooling fluid within said casing, spaced apart portions of said apparatus immersed in said fluid being at different electrical potentials and being subject to arcing therebetween incident to a failure which may rapidly develop sufficient pressure within said casing to damage said casing, said aperture being of sufficient size to vent said casing before it is damaged from pressure incident to a large kva. arc, pressure responsive means including a double area cover for said aperture mounted for displacement in response to the pressure within said casing between a closed position, wherein said aperture is sealed, and an open position wherein said casing is vented through said aperture, one of said areas on said cover being exposed to the interior of said casing through said aperture when said cover is in closed position, first means including a pair of opposed continuous surfaces surrounding said one area for sealing said aperture and for isolating the other area on said cover from the interior of said casing when said cover is in closed position, one of said surfaces being on said cover and the other being on said casing, second means including a pair of opposed surfaces, one of which is on said cover and the other of which is on said casing, surrounding both of said areas on said cover for providing restricted communication between said other area and the ambient when said cover is in closed position, and resilient means for holding said cover in closed position, said pressure responsive means being operable in response to a predetermined pressure within said casing to open the seal at said first means and expose said other area to pressure within said casing before said opposed surfaces of said second means are separated to fully vent said other area to the ambient, whereby both of said areas on said cover are exposed to the internal pressure within said casing and said cover is rapidly actuated to open position.

6. In an electrical apparatus, in combination, a casing having an aperture therein, a cooling and insulating fluid within said casing, spaced apart portions of said apparatus being at different electrical potentials and being subject to arcing therebetween incident to a failure which may rapidly develop suflicient pressure within said casing to damage said casing, said aperture being of suflicient size to vent said casing before it is damaged from pressure incident to a large kva. arc, pressure responsive means including a cover for said aperture, said cover having inner and outer continuous flanges extending toward the interior of said casing and dividing said cover into inner and outer areas, a first continuous surface on said casing surrounding said aperture and being opposed to and in sealing engagement with said inner flange when said cover is in closed position, whereby said inner area is exposed to pressure within said casing and said outer area is isolated therefrom when said cover is in closed position, means including a second continuous surface on said casing opposite said outer flange for providing restricted communication between said outer area and the ambient when said cover is in closed position, said pressure responsive means being responsive to a predetermined pressure within said casing to open the seal at 13 said inner flange and exposed both said inner and outer areas of said cover to the internal pressure within said casing before said second continuous surface and said outer flange are separated to fully vent said outer area to the ambient, whereby said cover is rapidly actuated to open position wherein said casing is exhausted to the ambient, and resilient means biasing said cover into closed position and returning said cover to closed position after pressure within said casing has decreased to a second predetermined value.

7. In an electrical transformer, in combination, a casing having an opening therein, a cooling and insulating fluid in said casing, a core and coil immersed in said fluid, an annular body portion surrounding said opening and being mounted on said casing, said annular body portion having a radially inward extending flange terminating in a continuous rim defining an aperture in register with said opening, pressure responsive means including a cover for said aperture, said cover having continuous, depending inner and outer flanges defining inner and outer areas on said cover, said rim telescopingly overlapping and forming a seat for said inner flange when said cover is in closed position, means including said outer flange in telescopingly overlapping relation with the outer periphery of said body portion to provide restricted communica tion between said outer area and the ambient when said cover is in closed position, and resilient means for holding said cover in closed position, said pressure responsive means being operable in response to a predetermined pressure within said casing to expose said outer area to the interior of said casing before said outer flange is moved out of telescopingly overlapping relationship to vent said outer area to the ambient, whereby both said inner and outer areas are exposed to internal pressure within said casing and said cover is rapidly actuated away from closed position to exhaust said casing to the ambient.

8. In electrical apparatus, in combination, a casing having an opening therein, a cooling and insulating fluid in said casing, a body portion surrounding said opening and rigidly mounted on said casing, said body portion having a radially inward extending flange terminating in a continuous rim defining an aperture in register with said opening, a cover for said aperture, said cover having continuous, depending inner and outer flanges defining inner and outer areas on said cover, said rim and said inner flange defining a first pair of telescopingly overlapping surfaces when said cover is in closed position and said outer flange and the outer periphery of said body portion defining a second pair of telescopingly overlapping surfaces when said cover is in closed position, a first continuous groove in one of said first pair of surfaces and a second continuous groove in one of said second pair of surfaces, a first continuous resilient gasket within said first groove compressed between said first pair of surfaces and a second continuous resilient gasket within said second groove compressed between said second pair of surfaces when said cover is in closed position, resilient means disposed within said body portion for biasing said cover into closed position, means for providing restricted communication between said outer area and the ambient when said cover is in closed position, said cover being displaceable against the force of said resilient means in response to a predetermined pressure within said casing to move said first gasket and the other of said first pair of surfaces out of engagement, the displacement of said cover necessary to move said first pair of surfaces out of engagement being less than the displacement necessary to disengage said second gasket from the other of said second pair of surfaces, whereby both said inner and outer areas of said cover are exposed to internal pressure within said casing and said cover is rapidly actuated to open position wherein said casing is vented.

gene a 9. in an electrical transformer, in combination, a casing having an opening therein, a cooling and insulating liquid in said casing, a core and coil immersed in said liquid, a gas filling said casing above said liquid and being in communication with said opening, pressure relief means including a body member surrounding said opening and rigidly mounted on said casing, said body member having a radially inward extending flange terminating in a continuous rim defining an aperture in register with said opening, said pressure relief means also including a cover member for said aperture, said cover member having continuous, depending inner and outer flanges defining inner and outer areas on said cover member, said rim sealingly engaging and forming a seat for said inner flange when said cover member is in closed position, said outer flange being in sealing engagement with the outer periphery of said body member when said cover member is in closed position, and resilient means disposed within said body member for biasing said cover member into closed position, said pressure relief means being operable in response to a predetermined pressure within said casing to expose said outer area to the interior of said casing before said outer flange is moved out of sealing relation with said body member, whereby both said inner and outer areas are exposed to internal pressure within said casing and said cover member is rapidly actuated toward open position wherein said casing is exhausted to the atmosphere, one of said members having at least one orifice therethrough providing communication between said outer area and the ambient, whereby gases trapped between said inner and outer flanges beneath said cover member cannot exert an initial outward pressure on said cover.

10. In an electrical apparatus, the combination of a casing having an aperture therein, a cooling fluid in said casing, spaced apart portions of said apparatus being at different electrical potentials, whereby arcing therebetween incident to a fault may rapidly develop sufficient pressure within said casing to damage said casing, a body member mounted on said casing and having an opening therein in register with said aperture and of sufficient size to vent said casing before it is damaged from pressure incident to a large kva. arc, a pressure responsive member adapted when in closed position to seal said opening and being mounted for displacement toward an open position to vent said casing to the atmosphere in response to a predetermined pressure within said casing, resilient means for biasing said pressure responsive member into closed position, the portion of said pressure responsive member disposed toward the interior of said casing having an inner area and an outer area both of which extend transversely of said opening, said opening forming a seat for said inner area when said pressure responsive member is in closed position, first means for sealing between said pressure responsive member and said body member including a first pair of continuous, telescopingly overlapped surfaces surrounding said inner area, one of said surfaces being on said pressure responsive member and the other being on said body member and said surfaces overlapping a first predetermined distance when said pressure responsive member is in closed position, second means for sealing between said pressure responsive member and said body member including a second pair of continuous, telescopingly overlapped surfaces surrounding said outer area, one of said second pair of surfaces being on said body member and the other being on said pressure responsive member and said second pair of surfaces overlapping a second predetermined distance which is greater than said first predetermined distance, whereby when said pressure responsive member is displaced outward in response to said internal pressure within said casing the seal at said first sealing means is broken and said outer area is exposed to internal pressure within said casing before the seal at said second sealing means is broken, one of said members having at least one orifice therethro ugh providing restricted communication between said outer area and the ambient when said pressure responsive member is in closed position, said biasing means returning said pressure responsive member to closed position when the pressure is relieved within said casing.

11. In electrical apparatus, in combination, a casing having an aperture therein, a cooling and insulating fluid within said casing, spaced apart portions of said apparatus being at different electrical potentials and being subject to arcing therebetween incident to a failure which may rapidly develop sufiicient pressure within said casing to damage said casing, pressure responsive means including a double area cover for said aperture, said aperture being of sufficient size to vent said casing before it is damaged from pressure incident to a large kva. fault and providing a seat for one of the areas of said cover, said one area being in communication with the interior of said casing and the other area being isolated therefrom when said cover is closed, said means being responsive to a predetermined internal pressure Within said casing to expose both areas of said cover to the internal pressure within said casing, resilient means for biasing said cover into closed position, movement of said cover away from said closed position permitting escape of gas from within said casing, said cover being returned to its closed position by said biasing means after the pressure within the casing drops to a second predetermined value, and alarm means responsive to displacement of said cover for giving a continuous indication after said cover has been displaced away from closed position, said alarm means being resettable from the exterior of the casing without de-energizing said electrical apparatus.

12. In electrical apparatus, the combination of a casing housing said electrical apparatus and having an opening therein, a pressure responsive member normally closing said opening, resilient means for biasing said pressure responsive member into closed position, said memher being displaceable away from said closed position against the force of said resilient biasing means to vent said casing in response to a predetermined pressure within said casing, said member and said casing having a pair of opposed continuous surfaces parallel to the direction of displacement of said member and surrounding an area on said member exposed to the interior of said casing, a continuous groove in one of said surfaces, and a continuous resilient gasket within said groove and compressed against the other of said surfaces when said member is in closed position, said biasing means returning said member to closed position when the pressure within said casing is relieved, whereby variations in the characteristics of said gasket and the degree of compression thereof will not substantially affect the calibration of said pressure responsive member.

13. In electrical apparatus, the combination of a casing housing said electrical apparatus and having an opening therein, a double area pressure responsive member normally closing said opening and being displaceable away from said closed position to vent said casing in response to a predetermined pressure within said casing, resilient means for biasing said member into closed position, one of the areas on said cover being exposed to the interior of said casing when said cover is in closed position, said member and said casing having a first pair of opposed continuous surfaces parallel to the direction of displacement of said member and surrounding said one area, a continuous groove of trapezoidal cross section in one of said first pair of surfaces, a continuous resilient gasket within said groove and compressed against the other of said surfaces when said member is in closed position, said member and said casing having a second pair of opposed continuous surfaces parallel to the direction of displacement of said member and surrounding both 16 areas on said member, a continuous groove of trapezoidal cross section in one of said above pair of surfaces, and a continuous resilient gasket within said groove in said one of said second pair of surfaces, said biasing means returning said member to closed position when the pressure within said casing is relieved.

14. In electrical apparatus, in combination, a casing for said apparatus, said casing having an opening therein, a cover normally closing said opening, resilient means for biasing said cover into closed position, said cover being movable away from said closed position against the force of said resilient biasing means to vent said casing in response to a predetermined pressure within said casing, said cover and said casing having a pair of opposed continuous surfaces surrounding an area on said cover exposed to the interior of said casing, a continuous groove in one of said surfaces, a continuous resilient gasket within said groove compressed against the other of said surfaces when said cover is in closed position and said cover being in direct engagement with said casing when said gasket is so compressed, said biasing means returning said cover to closed position when the pressure within said casing is relieved and said biasing means being sufiiciently strong to hold said cover in direct engagement with said casing over the entire operating range of pressure which occur within said casing during the normal operation of said apparatus.

15. In electrical apparatus, the combination of a casing having an opening in a wall thereof and a continuous portion substantially at right angles to said wall and surrounding said opening, a cooling fluid in said casing, spaced apart portions of said apparatus being at different electrical potentials and being subject to arcing therebetween incident to a failure which may rapidly develop sufficient pressure within said casing to damage said casing, said opening being of sufficient size to vent said casing before it is damaged from pressure incident to a large kva. arc, and pressure responsive means including a cover mounted in said opening for displacement in response to the pressure in said casing, said cover having a continuous portion parallel to and opposite said continuous portion on said casing when said cover is in closed position, resilient, continuous gasket means between said opposed continuous portions on said cover and said casing and compressed therebetween in a direction at right angles to the direction of the force exerted on said cover by said pressure Within said casing for sealing between said cover and said casing when said cover is in closed position, whereby variations in the characteristics of said gasket means and the degree of compression thereof will not materially aflect the calibration of said pressure responsive means, and resilient means for biasing said cover into closed position, said cover being actuated from said closed position against the force of said resilient means in response to a predetermined pressure within said casing, said biasing means returning said cover to said closed position when the pressure within said casing is relieved.

References Cited in the file of this patent UNITED STATES PATENTS 162,831 Kunkle May 4, 1875 1,549,665 Hill Aug. 11, 1925 1,687,641 Aalborg Oct. 16, 1928 2,431,769 Parker Dec. 2, 1947 FOREIGN PATENTS 286 Great Britain Nov. 3, 1904 of 1904 

